1. Field of the Invention
The present inventin relates to an oil strainer for an automatic transmission and to the automatic transmission using the oil strainer, and, in particular, to the structure of a strainer for an automatic transmission having a channel leading from a valve body directly to an oil pump and a channel leading from an oil sump through a filter element to an oil pump, and to the structural arrangement of the oil strainer in the automatic transmission.
2. Description of the Prior Art
Conventionally, for example, in providing an oil strainer as shown in Japanese Laid Open Patent No. Sho-58-44006, a metal sheet is subjected to a stamping process to form a dish-shaped base member with a peripheral edge shoulder section and a flat wall section having a large number of supporting bosses. Mounted onto this dish-shaped base member is a filter element having a fiber mesh securely mounted to a filter frame with a plurality of ribs.
This strainer has a flat shape and is housed in an oil pan secured on the bottom of a transaxle casing. The oil in the oil pan is sucked into the oil strainer through a filter element, passes through a flow channel made up of the spaces formed from the ribs of the filter frame and the supporting bosses of the dish-shaped base member and is conducted to a discharge port.
However, such an oil strainer as described above is made in a flat form, and the flow channel is small because of the space between the filter element and the dish-shaped base member, so that the volume of oil sucked into the pump is restricted. In addition, the ribs and supporting bosses are present in the narrow flow channel, so the resistance in the flow channel is also high. It is also easy for large pieces of foreign matter to accumulate near the discharge port, so that when the flow resistance becomes high from blinding of the filter element and the like, the filter element bends from the high suction pressure, causing the filter element to be tightly pressed to the walls of the dish-shaped base member, further restricting the flow channel.
In addition, since this type of oil strainer is positioned on the bottom of the oil pan, it is necessary to have a special type of securing means, such as a large number of bolts and the like, to secure it to the casing, and an oil channel to the oil pump is required. Assembly is a problem, and at low temperatures, resistance in the pipeline increases causing line pressure drop.